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Entry  Wed Apr 4 12:52:33 2018, awade, Summary, Drawings, Drawings: 2in windows for vacuum can flang and clamp VacuumViewportDesignNotebook.ipynb.zipTwoViewPort_10inchBlankFlange_Modified_v1.PDFTwoInchViewportRetainingRing_v1.PDF2inchWindowAssemb_3degangling_0.5degWedge.PDFSolidWorksFiles_NewVacCanFlanges.zip
    Reply  Wed Apr 11 16:06:31 2018, awade, Summary, Drawings, Drawings: 2in windows for vacuum can flang and clamp 2inchWindowAssemb_explodedView_50frameswtblur_medium.gif
       Reply  Fri Apr 13 11:34:09 2018, awade, Summary, Drawings, Drawings: 2in windows for vacuum can flang and clamp, steve's comments 
Message ID: 2163     Entry time: Wed Apr 4 12:52:33 2018     Reply to this: 2168
Author: awade 
Type: Summary 
Category: Drawings 
Subject: Drawings: 2in windows for vacuum can flang and clamp 

I've drafted up some new drawings for new view ports into our vacuum can.  We have 8 inch windows at the moment with AR coating and surface quality that could be better.  Given the criticalness of scatter we plan to replace the windows with a set of four 2 inch AR-AR coated windows; smaller windows will also reduce the exposed thermal surfaces to outside, thus improving thermal isolation of the cavity shields from the outside. 

Specifications of view port windows

Planned spec for the windows is 10-5 scratch-dig, lambda/10 flatness and AR of R<=0.25 % with a 30 arc second wedge. I've contacted a number of vendors for quotes. As we can't list prices publicly here I've made a summary page HERE with prices and specs from vendors that responded.


Design of flange and retaining ring

To deal with any residual beams from the surfaces of the window we will angle the windows at 3 degrees to normal by insetting them into the flange: centered beams on the optic that are normal the flange face will then be reflected at 3 degrees (52 mrad) which at a distances of roughly 20 cm will be 1 cm offset from main beam, this is plenty of space to fit in a beam dump. This design may present challenges for machining.  I'm pretty sure the groves can't just be milled, they need lathing with certain tool specifications to ensure there are no radially oriented scratches that might compromise the seal. The flange is a bulky piece, we need to ask a machinist what can be done.  The angled inset is the way to go, even if its a little harder to make.

I based these drawings partly off Johannes's design (see Cryo:1456). The particulars of their design meant that I couldn't reuse the parts, but I took cues from the design features (that made things much faster to draft). I rebuilt the flange parts and retaining ring with a more parametric design.  Within the part files dimensions are defined in a way that makes it easy to change wedging angles, groove dimensions and sinking depth of the optic.  Hopefully this will help future users adapt them to their needs quickly.  Johannes also had a good reference for view port designs Abbot & Scace in J. Vac. Sci. Technol. A 28, 573 (2010).  One thing that isn't addressed much in the literature is the extra loading applied on the inner o-rings when 1 atm of pressure is applied (vacuum).  For a 2 in^2 window area. 1 atm is about 20.9 kg of pressure (8.6 lb/in of oring). For 70 duro viton o-rings the additional compression looks to be on order of 7 %.  Provided there enough clearance is left between glass and metal, this shouldn't me much of an additional error: it just something to keep in mind for tolerances of the parts.

Drawings are attached below along with an ipython notebook used for the calculation of various dimensions (this can also be found in a gist on GitHub). A zip contains all the parts, drawings and assemblies.

The basic design criteria are for a 2.0" view port optic, wedge by 30 arc min, that is 9.35 mm on its thickest edge.  Clearance of 3/128" are made around the perimeter and 0.5 mm between the faces and metal of the flange and retaining ring.  The o-ring I selected is #130 which has ID of 1.612" and thickness (toroidal diameter) of 0.103".  With a set compression ratio of 0.72 for the o-ring it is possible to make an o-ring groove just deeper than half the o-ring thickness. An o-ring any smaller will be more than double any possible groove depth, which would be bad as would will pop out during assembly. Dimensions of the o-ring groove were chosen according to rules outlined in Abbot & Scace. I think at a 2" diameter it is a little too small to make dove tale o-ring grooves, so the inner diameter of the groove was made 2% larger than the o-ring so that it will be under a little tension while the unit is assembled.  The outer edge of the o-ring groove is 2 mm from the edge of the optic, this should be enough space for the o-ring to be clear of any edge imperfections. Calculations for all these quantities can be found in the ipython notebook.  

The rest of the dimensions were adjust accordingly to reach the correct clearances, an optic angle of 3 degrees to the front of the flange and leave 2 mm protruding on the lowest side of the optic.  The two windows are oriented about the center line of the 10" flange blank, spaced 3.0" apart.  The angle is currently set to point outwards on the outside  of the tank; this orientation might be wrong for our needs as we want to primarily dump light inside the tank and it would be easier to put the beam dumps not between the two beams.

I need to stop fiddling with this now.  It would be good if someone could look over the drawings and any raise issues. 


Attachment 1: VacuumViewportDesignNotebook.ipynb.zip  4 kB  Uploaded Fri Apr 6 02:03:15 2018
Attachment 2: TwoViewPort_10inchBlankFlange_Modified_v1.PDF  106 kB  Uploaded Fri Apr 6 02:11:02 2018  | Hide | Hide all
Attachment 3: TwoInchViewportRetainingRing_v1.PDF  74 kB  Uploaded Fri Apr 6 02:11:49 2018  | Hide | Hide all
Attachment 4: 2inchWindowAssemb_3degangling_0.5degWedge.PDF  73 kB  Uploaded Fri Apr 6 02:11:58 2018  | Hide | Hide all
Attachment 5: SolidWorksFiles_NewVacCanFlanges.zip  20.389 MB  Uploaded Fri Apr 6 02:13:25 2018
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